1. Field of the Invention
The present invention relates to a two-way optical communication apparatus for performing transmission/reception of a signal through the medium of a light beam by utilizing space as a transmission path, and more particularly, to an optical communication apparatus which exhibits excellent transmission efficiency. The excellent transmission efficiency is achieved by using a phase plate to convert the polarized state of a polarized light beam.
2. Description of the Related Art
Two-way optical communication apparatuses that utilize a phase plate to convert the polarized state of a polarized light are known in the art.
FIGS. 6(A) and 6(B) illustrate an example of such an apparatus. This apparatus is disclosed in U.S. Pat. No. 4,199,226. The apparatus utilizes a source of a linearly polarized laser beam and a quarter-wave plate, and is used for two-way communications in which the laser beam is transmitted through a transmission space as circularly polarized light and in which that laser beam is received on a light-receiving element as a linearly polarized light. A beam emitted from a laser source 101 enters a lens 102. The lens 102 creates a beam made up of parallel rays of light from the incident light. The resultant beam made up of parallel rays of light enters a polarization beam splitter 103. At that time, a joining surface of the polarization beam splitter 103 passes most of the linearly polarized laser beam therethrough. Thereafter, an etalon 104 narrows the spectral bandwidth of the laser beam. The laser beam emitted from the etalon 104 passes through a laser oscillation-stabilizing returning-light generating element 105, and then enters a quarter-wave plate 106. The optical axis of the quarter-wave plate 106 is located at a position that allows the incident linearly polarized laser beam to emerge therefrom as circularly polarized. Thereafter, the laser beam is transmitted toward a remote party through a beam expander 107.
A circularly polarized laser beam 108, which is received from the remote party, enters the quarter-wave plate 106 through the beam expander 107. The laser beam emerges from the quarter-wave plate 106 as linearly polarized light that can be reflected by the joining surface of the polarization beam splitter 103. Therefore, the joining surface of the polarization beam splitter 103 passes most of the laser beam. The laser beam that has been reflected by the polarization beam splitter 103 passes through a polarization filter 108, which produces a linearly polarized light having a larger polarization ratio. After the laser beam has passed through an interference filter 109 for removing external light, it is received by a light-receiving element 112 through a lens 110.
FIG. 7 illustrates another example of the above-described type of apparatus. This apparatus is disclosed in Japanese Patent Laid-Open No. Hei 2-198234. The apparatus utilizes a source of a linearly polarized laser beam and a half-wave plate, and is designed to perform two-way communications by transmitting the laser beam as a linearly polarized light at any point of the space through which the laser beam passes.
A light 221, which is to be transmitted is emitted from a light transmitter 202. A polarization beam splitter 212 passes most of the light 221 therethrough. The light beam that has passed through the polarization beam splitter 212 passes through a polarization direction rotating device 201, a mirror angle driving mechanism 209 and then a beam expander 211, and is then transmitted.
A received light 222 passes through the beam expander 211 and then the mirror angle driving mechanism 209. A control circuit 205 controls mirror angle driving mechanism 209. An angle error detector 204 detects a status of a light reception and the control circuit 205 controls the driving mechanism 209 so that the status of the light reception will be at a most preferable level. Thereafter, the direction of polarization of the received light 222 is rotated such that it is perpendicular to the direction of polarization of the light to be transmitted 202. The joining surface of the polarization beam splitter 212 passes most of the received light emerging from the polarization direction rotating device 201. After the light beam is divided by a beam splitter 213, the divided beams enter a light receiver 203 and the angle error detector 204, respectively. The polarization direction rotating device 201 is made up of a half-wave plate 208 and a half-wave plate driving portion 207, and is designed to freely set the direction of polarization of the beam by rotating the half-wave plate 208 by, for example, a motor. Control of the rotating operation by the polarization direction rotator 201 may be performed by using the data which is input to a control signal processing circuit 210 from an external circuit. Alternatively, control of the rotating angle of the polarization direction rotating device 201 may be performed by sending, to the half-wave plate driving portion 207 of the polarization direction rotating device 201 through a polarization direction rotating device driving control circuit 206, a signal representing the amount or direction of the rotation, which is obtained by identifying the received signal from the light receiver 203 or the angle error detector 204 by means of the control signal processing circuit 210.
In the apparatus shown in FIG. 6(A). there is no clear description regarding the optical axis of the crystal of the quarter-wave plate 106. In addition, there is no description about the coincidence of the light communication apparatuses of the two parties associated with communications.
If the apparatus shown in FIG. 6(A) is used to perform communications with a remote apparatus, which has the same structure as that of the apparatus shown in FIG. 6(A) and which employs a quarter-wave plate whose optical axis is inclined by 45 degrees to the left from the upward direction of the vertical direction as viewed from the front side of the apparatus so as to allow the incident light to emerge therefrom as circularly polarized, a laser beam whose plane of polarization 8a is coincident with the vertical direction (in a Y-axis direction in FIGS. 6(A) and 6(B)) passes through the quarter-wave plate whose optical axis 9a is inclined by 45 degrees to the right from the vertical direction. The laser beam emerging from the quarter-wave plate as a circularly polarized light 10a propagates in the transmission space, as shown in FIG. 6 (B). This light beam enters the remote apparatus. Since an optical axis 9b of a quarter-wave plate 5b, of the remote apparatus, is inclined by 45 degrees to the left from the vertical direction, as shown in FIG. 6 (B), a plane of polarization 11a of the laser beam, which has passed through the quarter-wave plate 9b, is coincident with the vertical direction, and the plane of polarization 11a of the light beam that is received by the remote apparatus hence coincides with the plane of polarization 8b of the light beam transmitted from the remote apparatus. Therefore, the polarization beam splitter 103 cannot separate the polarized light, and most of the received light enters the laser diode 101, making reception impossible.
Further, the two-way optical communication apparatus shown in FIG. 7 is used under the condition that the plane of polarization of a received light is unknown. Consequently, control of the rotation of the half-wave plate is required. This makes the structure of the apparatus complicated and thus increases production cost.